Investigating an Outbreak
Principles of Epidemiology Lecture 8
Dona Schneider, PhD, MPH, FACE
What is an outbreak?
An epidemic or an outbreak exists when there are more cases of a particular disease than expected in a given area, or among a specific group of people, over a
particular period of time.
Epidemiology (Schneider)
Endemic vs. Epidemic
No. of Cases of a Disease
Endemic
Time
Epidemic
Epidemiology (Schneider)
Why investigate outbreaks or epidemics?
Control and prevention Severity and risk to others Research opportunities to gain additional knowledge Training opportunities
Program considerations
Public, political, or legal concerns
Epidemiology (Schneider)
Step 1: Verify the outbreak
Determine whether there is an outbreak – an excess number of cases from what would be expected Establish a case definition
Non-ambiguous Clinical / diagnostic verification
Person / place / time descriptions
Identify and count cases of illness
Epidemiology (Schneider)
Step 2: Plot an Epidemic Curve
Graph of the number of cases (y-axis) by their date or time of onset (x-axis)
Interpreting an epidemic curve
Overall pattern: increase, peak, decrease
Type of epidemic? Incubation period? Unrelated? Early or late exposure? Index case? Secondary cases?
Outliers:
Epidemiology (Schneider)
Vector-borne Disease
• Starts slowly • Time between the first case and the peak is comparable to the incubation period. • Slow tail
Point Source Transmission
• This is the most common form of transmission in foodborne disease, in which a large population is exposed for a short period of time.
Continuing Common Source or Intermittent Exposure
• In this case, there are several peaks, and the incubation period cannot be identified.
Salmonellosis in passengers on a flight from London to the United States, by time of onset, March 13--14, 1984
Source: Investigating an Outbreak, CDC
Legionnaires' Disease By date of onset, Philadelphia, July 1-August 18, 1976
Source: Investigating an Outbreak, CDC
Foodborne Outbreak (Propagated)
Source: CDC, unpublished data, 1978
Step 3: Calculate attack rates
Attack rate = (ill / ill + well) x 100 during a time period If there is an obvious commonality for the outbreak, calculate attack rates based on exposure status (a community picnic)
If there is no obvious commonality for the outbreak, calculate attack rates based on specific demographic variables (hepatitis cases in a community)
Epidemiology (Schneider)
Step 4: Determine the source of the epidemic
If there is an obvious commonality for the outbreak, identify the most likely cause and investigate the source to prevent future outbreaks
If there is no obvious commonality for the outbreak, plot the geographic distribution of cases by residence/ work/school/location and seek common exposures
Epidemiology (Schneider)
Step 5: Recommend control measures
Control of present outbreak Prevention of future similar outbreaks
Epidemiology (Schneider)
The vast majority of outbreaks are food-borne
Foodborne Disease Outbreak
An incident in which (1) two or more persons experience a similar illness after ingestion of a common food, and (2) epidemiologic analysis implicates the food as the source of the illness
Intoxication – ingestion of foods with
in tissues of certain plants (Jimpson Weed) and animals (seal liver)
Metabolic Toxicants found
products (toxins) formed and excreted by microorganisms while they multiply (botulinum toxin)
Poisonous
substances introduced during production, processing, transportation or storage (chemicals, pesticides)
Foodborne Disease Outbreak (cont.)
Infections – Caused by the entrance of pathogenic microorganisms into the body and the reaction of the body tissues to their presence or to toxins they generate within the body Rule of thumb – but not law
Intoxicants are rapid onset, no fever
Toxins in the stomach produce vomiting Toxins in the intestines produce diarrhea
Infections produce fever
Types of Foodborne Contamination
Physical
Glass, metal fragments, tacks, dirt, bone, etc.
Chemical
Pesticides, cleaning compounds, poisonous metals, additives and preservatives
Biological
Bacteria, viruses, fungi, yeast, molds, parasites, poisonous fish and plants, insect and rodents
Epidemiology (Schneider)
Bacterial Requirements
Food: Most bacteria require what is known as potentially hazardous food
Milk or milk products, eggs, meat, poultry, fish, shellfish, crustaceans, raw seed sprouts, heat treated vegetables and vegetable products (fruits?)
Generally high protein, moist foods
Epidemiology (Schneider)
Bacterial Requirements (cont.)
Water: Bacteria require moisture to thrive
The water activity (Aw) is the amount of water available in food The lowest Aw at which bacteria will grow is 0.85
Most potentially hazardous foods have a water activity of 0.97 to 0.99
pH: Best growth at neutral or slightly acidic pH
Potentially hazardous foods have a pH of 4.6 – 7.0
Epidemiology (Schneider)
Bacterial Requirements (cont.)
Temperature: The danger zone for potentially hazardous foods is 45 to 140 degrees Fahrenheit
This is the zone where most bacterial growth occurs
Time: Potentially hazardous foods must not be allowed to remain in the danger zone for more than 4 hours Oxygen: Some bacteria require oxygen while others are anaerobic and others are facultative
Epidemiology (Schneider)
Major Causes of Foodborne Disease
Improper cooling of foods Improper cooking of foods Improper reheating of foods Improper holding temperature of foods Cross contamination
Infected food handlers, poor employee hygiene
Epidemiology (Schneider)
0F
Temperature and Bacteria Control
Canning temperatures for low-acid vegetables, meat, and poultry in pressure canner
Canning temperatures for fruits, tomatoes, and pickles in waterbath canner
250 240 212
Water boils Most bacteria destroyed
165
No growth, but survival of some bacteria
140 DANGER ZONE
Some bacterial growth; many bacteria survive
125 120 98.6
60
Hottest temperature hands can stand
Extreme DANGER ZONE. Rapid growth of bacteria and production of poisons by some bacteria
Body temperature – ideal for bacterial growth Some growth of food poisoning bacteria may occur Slow growth of some bacteria that cause spoilage Water freezes
Growth of bacteria is stopped, but bacteria level before freezing remains constant and not reduced
45 40
32
0 - 20
Keep frozen foods in this range
Source: Keeping Food Safe to Eat, USDA
Bacterial Growth Curve
Stationary Phase
Log Phase Number of Cells
Decline Phase
Lag Phase
Time
Epidemiology (Schneider)
Effect of Temperature in Salmonella Growth
Number of Salmonella per gram
50oF (10o C) 95oF (35o C)
44oF (6.7o C)
42oF (5.5o C)
1
2
3
4
5
Days
Epidemiology (Schneider)
Incubation Periods
2-4 hours
12 hours
Staphylococcus aureus
Clostridium perfringens
Cooked ham, meat, eggs, sauces and gravies
Cooked meats, gravy
12-36 hours
12-36 hours
Salmonella*
Clostridium botulinum
Meat, poultry, eggs
Canned foods, smoked fish
12 hours
24-48 hours
Vibrio parahemolyticus*
Shigella*
Raw fish, shellfish
Contaminated by carrier, not foodborne
* Fever
National Data on Etiology of Foodborne Illness
Agent
Bacteria (40 agents) Salmonella Staph. aureus 68.7% 25.0% 12.7%
Clostridium perfringens Clostridium botulinum
Viral (11 agents) Parasites (31 agents) Fungal (16 agents)
10.0% 9.5%
9.4% 0.5% 1.8%
Plants (36 agents) Fish (28 agents) Chemicals (28 agents)
12.3% 7.3%
Investigating an Epidemic: Oswego, NY
On April 19, 1940, the local health officer in the village of Lycoming, Oswego County, New York, reported the occurrence of an outbreak of acute gastrointestinal illness to the District Health Officer in Syracuse. Dr. A. M. Rubin, epidemiologist-in-training, was assigned to conduct an investigation.
When Dr. Rubin arrived in the field, he learned from the health officer that all persons known to be ill had attended a church supper the previous evening, April 18. Family members who had not attended the church supper had not become ill. Accordingly, the investigation was focused on the circumstances related to the supper.
Source: CDC
Interviews regarding the presence of symptoms, including the day and hour of onset, and the food consumed at the church supper, were completed on 75 of the 80 persons known to have been present. A total of 46 persons who had experienced gastrointestinal illness were identified. Q: Is this an Epidemic? Endemic for the region? Due to seasonal variation? Due to random variation?
Epidemiology (Schneider)
Select the correct case definition and find the error in the others:
1.
All participants in the Oswego church supper held in the basement of the church in Lycoming, Oswego County, New York, on April 18, 1940, between 6:00 PM and 11:00 PM; whether they attended church or not; whether they participated in food preparation, transport, or distribution or not; whether they ate or not. Persons who developed acute gastrointestinal symptoms within 72 hours of eating supper on April 18, 1940, and who were among attendees of the Lycoming, Oswego Church supper.
Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940.
2.
3.
Select the correct case definition and find the error in the others:
1.
All participants in the Oswego church supper held in the basement of the church in Lycoming, Oswego County, New York, on April 18, 1940, between 6:00 PM and 11:00 PM; whether they attended church or not; whether they participated in food preparation, transport, or distribution or not; whether they ate or not. Persons who developed acute gastrointestinal symptoms within 72 hours of eating supper on April 18, 1940, and who were among attendees of the Lycoming, Oswego Church supper.
Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940.
2.
3.
Select the correct case definition and find the error in the others:
1.
All participants in the Oswego church supper held in the basement of the church in Lycoming, Oswego County, New York, on April 18, 1940, between 6:00 PM and 11:00 PM; whether they attended church or not; whether they participated in food preparation, transport, or distribution or not; whether they ate or not. Missing definition of sickness Persons who developed acute gastrointestinal symptoms within 72 hours of eating supper on April 18, 1940, and who were among attendees of the Lycoming, Oswego Church supper. CORRECT
Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940. Did not specify that they went to the dinner
2.
3.
Incidence of Cases of Diarrhea Among People Attending Lycoming,Oswego Church Supper, June 1940
The supper was held in the basement of the village church. Foods were contributed by numerous
members of the congregation. The supper began at 6:00 PM and continued until 11:00 PM. Food was spread out upon a table and consumed over a period of several hours.
Epidemiology (Schneider)
Church Supper Menu
Main Dishes • • • • • • • • • • • • • • Baked ham Spinach Mashed potatoes Cabbage salad Fruit Salad Jello Rolls Brown Bread Cakes Vanilla Ice Cream Chocolate Ice Cream Milk Coffee Water
Side Dishes
Desserts
Beverages
Epidemiology (Schneider)
Which menu item(s) is the potential culprit? To find out, calculate attack rates. The foods that have the greatest difference in attack rates may be the foods that were responsible for the illness.
Epidemiology (Schneider)
Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940
Number of persons who ate Ill 29 specified item Well Total Attack rate (%) 17 46 Number of persons who did not eat specified item Ill 17 Well 12 Total 29 Attack rate %
Baked ham
Spinach
Mashed potato
26
23
17
14
43
37
20
23
12
14
32
37
Cabbage salad
Jello Rolls
18
16 21
10
7 16
28
23 37
28
30 25
19
22 13
47
52 38
Brown bread
Milk Coffee
18
2 19
9
2 12
27
4 31
28
44 27
20
27 17
48
71 44
Water
Cakes Ice cream (van)
13
27 43
11
13 11
24
40 54
33
19 3
18
16 18
51
35 21
Ice cream (choc)
Fruit salad
25
4
22
2
47
6
20
42
7
27
27
69
Epidemiology (Schneider)
Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940
Number of persons who ate Number of persons who did not eat specified item Ill 17 20 Well 12 12 Total 29 32 Attack rate % 59 62
specified item
Ill 29 26 Well 17 17 Total 46 43 Attack rate (%) 63 60
Baked ham Spinach
Mashed potato
Cabbage salad Jello
23
18 16
14
10 7
37
28 23
62
64 70
23
28 30
14
19 22
37
47 52
62
60 58
Rolls
Brown bread Milk
21
18 2
16
9 2
37
27 4
57
67 50
25
28 44
13
20 27
38
48 71
66
58 62
Coffee
Water Cakes
19
13 27
12
11 13
31
24 40
61
54 67
27
33 19
17
18 16
44
51 35
61
65 54
Ice cream (van)
Ice cream (choc) Fruit salad
43
25 4
11
22 2
54
47 6
80
53 67
3
20 42
18
7 27
21
27 69
14
74 61
Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940
Number of persons who ate
Ill Baked ham Spinach Mashed potato Cabbage salad Jello Rolls 29 26 23 18 16 21 specified item Well Total Attack rate (%) 17 17 14 10 7 16 46 43 37 28 23 37 63 60 62 64 70 57
Number of persons who did not eat specified item
Ill 17 20 23 28 30 25 Well 12 12 14 19 22 13 Total 29 32 37 47 52 38 Attack rate % 59 62 62 60 58 66
Brown bread
Milk Coffee
18
2 19
9
2 12
27
4 31
67
50 61
28
44 27
20
27 17
48
71 44
58
62 61
Water
Cakes Ice cream (van)
13
27 43
11
13 11
24
40 54
54
67 80
33
19 3
18
16 18
51
35 21
65
54 14
Ice cream (choc)
Fruit salad
25
4
22
2
47
6
53
67
20
42
7
27
27
69
74
61
Highlighted row indicates largest difference between attack rates
Attack Rate by Consumption of Vanilla Ice Cream, Oswego, New York; April 1940
Ill Ate vanilla ice cream? Yes No
Total
Well 11 18
29
Total 54 21
75
Attack Rate (%) 79.6 14.3
61.3
43 3
46
•
•
The relative risk is calculated as 79.6/14.3 or 5.6
The relative risk indicates that persons who ate vanilla ice cream were 5.6 times more likely to become ill than those who did not eat vanilla ice cream
Conclusion
An attack of gastroenteritis occurred following a church supper at Lycoming The cause of the outbreak was most likely contaminated vanilla ice cream
Epidemiology (Schneider)
Surveillance
Ongoing systematic collection, collation, analysis
and interpretation of data; and the dissemination of information to those who need to know in order
that action may be taken.
World Health Organization
Epidemiology (Schneider)
Purposes of Public Health Surveillance
Estimate magnitude of the problem
Determine geographic distribution of illnesses Portraying the natural history of disease Detect epidemic / Define a problem Generate hypotheses and stimulate research Evaluate control measures Monitor changes in infectious agents Detect changes in health practice Facilitate planning
CDC
Epidemiology (Schneider)
Passive Surveillance
Physicians, laboratories, and hospitals are given forms to complete and submit with the expectation that they will report all of the cases of reportable disease that come to their attention
Advantages: Inexpensive
Disadvantages: Data are provided by busy health professionals. Thus, the data are more likely to be incomplete and underestimate the presence of disease in the population
Epidemiology (Schneider)
Active Surveillance
Involves regular periodic collection of case reports by telephone or personal visits to the reporting individuals to obtain the data
Advantages: More accurate because it is conducted by individuals specifically employed to carry out the responsibility Disadvantages: Expensive
Epidemiology (Schneider)
Sentinel Surveillance
Monitoring of key health events, through sentinel sites, events, providers, vectors/animals
Case report indicates a failure of the health care system or indicates that special problems are emerging Advantages: Very inexpensive
Disadvantages: Applicable only for a select group of diseases
Epidemiology (Schneider)
Some Surveillance Programs
National Notifiable Diseases Surveillance System
http://www.cdc.gov/epo/dphsi/nndsshis.htm
Morbidity and Mortality Weekly Report (MMWR)
http://www.cdc.gov
Cancer Surveillance, Epidemiology and End Result (SEER)
http://www.seer.cancer.gov/
Epidemiology (Schneider)
“Good surveillance does not necessarily ensure
the making of right decisions, but it reduces the chances of wrong ones.”
Alexander D. Langmuir
NEJM 1963;268:182-191
Epidemiology (Schneider)